The recent development of optical devices of various kinds has led to renewed interest in easy, rapid methods of producing optical gratings of various kinds. Such gratings have a variety of uses including frequency selection, optical feedback-type devices and wavelength dispersion. Reliable methods of making gratings of high quality with close spacing and high sensitivity are highly desirable.
A particular device in which the use of optical gratings is important is the single-frequency semiconductor laser. The development of optical communication systems has led to a variety of laser structures of particular interest. Much of the motivation for this work has been toward increasing the information carrying capacity (bit-rate capacity) of optical fibers. Of particular interest is the fabrication of a single longitudinal mode (or enhanced mode) laser for use in a single mode optical communication system.
A particularly interesting laser structure for single longitudinal mode operation is the distributed feedback (DFB) laser structure. This structure has been described in a number of publications including H. Kogelnik et al, Applied Physics Letters, 18(4), 152-154 (1971); T. Matsuoka et al, Electronics Letters, 18(1), 27-28 (1982); and S. Akiba et al, Electronics Letters, 18, 77-78 (1982).
Current efforts in the production of a single longitudinal mode laser have been centered on the GaInAsP/InP laser in the 1.55 .mu.m wavelength region where silica fibers, exhibit minimum attenuation.
DFB lasers are fabricated by producing a periodic surface corrugation in close proximity to the active region. Feedback occurs by Bragg scattering from the grating corrugations and is thus distributed throughout the laser structure. The lasing wavelength is determined by the period of the DFB grating. For first-order feedback at 1.55 .mu.m in GaInAsP, a grating period of approximately 0.23 .mu.m is required. High uniformity and accurate periodicity are required so as to yield high frequency selectivity and good laser operating characteristics.
A variety of procedures has been used to fabricate the grating corrugations for such lasers. Generally, a photoresist or electron resist mask is used in the fabrication procedure. For example, a holographic or electron beam procedure is used to pattern the resist and an ion milling procedure or chemical etching procedure is used to produce the necessary grating. Such work has been described in a number of references including M. J. Beesley et al, Applied Optics, 9(12), 2720-2724 (1970); L. F. Johnson et al, Applied Optics, 17(8), 1165-1181 (1978); L. D. Westbrook et al, Electronics Letters, 18(20), 863-865 (1982); H. L. Garvin et al, Applied Optics, 12(3), 455-459 (1973); C. V. Shank et al, Applied Physics Letters, 23(3), 154-155 (1973); and T. Saitoh et al, Electronics Letters, 18(10), 408-409 (1982).
Highly desirable in a simpler and more reliable procedure for making highly periodic grating structures at a periodicity useful for commercially valuable lasers. Particularly desirable would be a direct writing procedure which would eliminate the need for a resist to be used. Such a maskless procedure has been discussed in the literature previously (see D. V. Podlesnik et al, Applied Physics Letters, 43(12), 1083-1085 (1983)).